Scientists have found one of human's earliest ancestors 12,000 feet deep under the ocean - a weird pink worm named after a doughnut.

The four-inch (10 cm) long Xenoturbella churro - named for its resemblance to the popular fried-dough pastry - is one of four species discovered 1,700 metre deep in the Gulf of California.

Previously Xenoturbeklla was only known from a single species found in the North Atlantic, puzzling biologists for almost six decades.

Now expeditions half a world away by scientists from Scripps Institution of Oceanography at California University in San Diego, the Western Australia Museum and the Monterey Bay Aquarium Research Institute (MBARI) have helped properly identify the mysterious creatures.

Scripps marine biologist Dr Greg Rouse said: "The findings have implications for how we understand animal evolution. By placing Xenoturbella properly in the tree of life we can better understand early animal evolution."

The animal's shifting evolution began when the first species, named Xenoturbella bocki, was found off the coast of Sweden in 1950.

It was classified as a flatworm in the 1990s and thought of as a simplified mollusc. In recent years, though, Xenoturbella has been regarded as either close to vertebrates and echinoderms - starfish - or as a more distant relative of its own.

Expets said that knowing where Xenoturbella come from is important to understand the evolution of organs such as guts, brains and kidneys in animals.

MBARI scientist Dr Robert Vrijenhoek, who led the deep-sea expeditions using remotely operated vehicles, said: "When Greg first spotted the worms gliding through a clam field in Monterey Bay, we jokingly called them purple socks."

The hunt for more of the worms continued for the next 12 years. By 2015, they had found four new species and collected analysis.

Scientists examined nearly 1,200 of the animal's genes to identify them conclusively as evolved members of bilaterally symmetrical animals - such as certain crustaceans and even butterflies - which are distinguished by having matching halves through a line down the centre.

The Xenoturbella also have only one body opening, the mouth. They have no brain, gills, eyes, kidneys or anus and now appear to be evolutionarily simple rather than having lost these features over time.

As in, they have evolved to a point that they can function without them.

This discovery also increases the diversity of this known species from one to five. The largest of the new species, Xenoturbella monstrosa, was found in Monterey Bay off the coast of California and the Gulf of California and measured 8 ins (20 cm) long.

The smallest, Xenoturbella hollandorum, also found in Monterey Bay, was only one inch (2.5 cm) long.

And the Xenoturbella profunda was discovered in a 12,139 foot (3,700 metre) deep hydrothermal vent in the Gulf of California.

Dr Rouse said: "I have a feeling this is the beginning of a lot more discoveries of these animals around the world."

The oldest human ancestor is a 500 million-year-old worm-like creature no longer than a thumb.

Pikaia gracilens is the most primitive known vertebrate and therefore the ancestor of all descendant vertebrates - including humans.